According to much of the scientific literature, dominance in social animals goes hand-in-hand with healthier lives. Yet leaders of the pack might not be healthier in all aspects, and according to a study published last week (February 26) in Scientific Reports, they are more at risk of parasite infection.

“While high-ranking animals often have the best access to food and mates, these advantages appear to come with strings attached,” says study coauthor Elizabeth Archie, a behavioral and disease ecologist at the University of Notre Dame, in an email to The Scientist. “These strings take the form of higher parasite exposure and susceptibility.”

Lower social status is usually linked to poorer health, according to previous studies. Animals towards the bottom of hierarchies have to struggle more for resources, and are often subjected to aggressive behavior from their superiors. In many species of birds, mice, and nonhuman primates, for instance, poorer physical condition is more common for subordinates. Female macaques of low social status, for example, have been shown to have lower bone density and an increased risk of developing inflammatory diseases.

Yet the relationship between social subordination and infectious disease risk hasn’t been clearly measured, according Archie and her coauthors. To look at the relationship between social status and one particular malady—parasite infections—they carried out a meta-analysis of 39 studies spanning 31 species, searching for patterns of parasitism.

In the majority of studies, those individuals in dominant positions—in particular, dominant males—were found to be more at risk of being infected. The effect was strongest in mammals, and in ordered hierarchical societies where social status is correlated with sexual activity.

These findings support two previous hypotheses about the links between social status and parasitism. One relates infection risk to resource access: exposure to infection is more common when animals feed and mate more. Dominant reindeer, for example, spend more time eating than subordinate individuals, and are more likely to become infected by nematodes. And greater sexual activity brings more risk of transmitted infections. Take, for instance, dominant feral cats, whose sexual proclivity increases the chances of developing Feline Immunodeficiency Virus.

The other hypothesis proposes a trade-off between reproductive effort and immunity to disease. In other words, those in dominant positions expend more energy on mating, and therefore invest less into costly immune defences.

“When you put it in the context [of these hypotheses], it does make a lot of sense,” says Jennifer Koop, a biologist at the University of Massachusetts-Dartmouth, who was not involved in the study.

Archie doesn’t think that individuals will deliberately opt for lower status in order to avoid infection. “High status comes with so many other advantages that the cost of a few more parasites might not be enough for individuals to shun high social status,” she says.

It’s also conceivable that there are benefits to both parasite and host in this relationship, says Nicole Mideo, an evolutionary biologist at the Univeristy of Toronto, who was not involved in the study. “The parasites are exploiting the resources of the host, so if you have a host that doesn’t get access to much food, then the parasite isn’t going to get access to much food,” she says.

This study mostly focused on parasitic worms, a limitation the researchers want to expand beyond. Additionally, the toll on dominant animals’ health of the increased risk of parasite infections was not explored. Mideo explains that there could be subtle advantages here, as research has shown worms can alter immune systems, and might protect against other infections. “It’s entirely possible that having worm infections does confer some sort of advantage in the context of other potential diseases,” she says.

Psychologists studied how 28 horses reacted to seeing photographs of positive versus negative human facial expressions. When viewing angry faces, horses looked more with their left eye, a behaviour associated with perceiving negative stimuli. Their heart rate also increased more quickly and they showed more stress-related behaviours. The study, published February 10 in Biology Letters, concludes that this response indicates that the horses had a functionally relevant understanding of the angry faces they were seeing. The effect of facial expressions on heart rate has not been seen before in interactions between animals and humans.

Amy Smith, a doctoral student in the Mammal Vocal Communication and Cognition Research Group at the University of Sussex who co-led the research, said: “What’s really interesting about this research is that it shows that horses have the ability to read emotions across the species barrier. We have known for a long time that horses are a socially sophisticated species but this is the first time we have seen that they can distinguish between positive and negative human facial expressions.”

“The reaction to the angry facial expressions was particularly clear — there was a quicker increase in their heart rate, and the horses moved their heads to look at the angry faces with their left eye.”

Research shows that many species view negative events with their left eye due to the right brain hemisphere’s specialisation for processing threatening stimuli (information from the left eye is processed in the right hemisphere).

Amy continued: “It’s interesting to note that the horses had a strong reaction to the negative expressions but less so to the positive. This may be because it is particularly important for animals to recognise threats in their environment. In this context, recognising angry faces may act as a warning system, allowing horses to anticipate negative human behaviour such as rough handling.”

A tendency for viewing negative human facial expressions with the left eye specifically has also been documented in dogs.

Professor Karen McComb, a co-lead author of the research, said: “There are several possible explanations for our findings. Horses may have adapted an ancestral ability for reading emotional cues in other horses to respond appropriately to human facial expressions during their co-evolution. Alternatively, individual horses may have learned to interpret human expressions during their own lifetime. What’s interesting is that accurate assessment of a negative emotion is possible across the species barrier despite the dramatic difference in facial morphology between horses and humans.”

“Emotional awareness is likely to be very important in highly social species like horses — and our ongoing research is examining the relationship between a range of emotional skills and social behaviour.”

The horses were recruited from five riding or livery stables in Sussex and Surrey, UK, between April 2014 and February 2015. They were shown happy and angry photographs of two unfamiliar male faces. The experimental tests examined the horses’ spontaneous reactions to the photos, with no prior training, and the experimenters were not able to see which photographs they were displaying so they could not inadvertently influence the horses.